1. Field of the Invention (Technical Field)
The present invention relates to silver-based alloy compositions for use as reflective, semi-reflective or highly reflective layers or coatings for use in optical data storage media, low emissivity glass, transparent conductive displays, electro-chromic mirrors, or other reflective or semi-reflective applications.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes. Each of the publications is incorporated herein by reference.
Silver and some of its alloys have been employed for many years as reflectors in thick or thin film applications. In thick film applications such as paints, they were applied to the back side of the substrate and normally laminated into an assembly. In thin films, silver has been used on both the front and back side of substrates and has been employed as a mirroring material for IR, laser and visual light applications.
In all applications, pure silver thin films required protective layers on top, or in some cases, below, to prevent degradation of the film. Silver also requires edgewise protection to inhibit corrosion at the edges in the film that slowly creep into the working surface area of the film.
Historically, gold or platinum group metals such as palladium or platinum have been added as a way of adding nobility to the silver. This has worked for many applications, but the added cost of gold or platinum group metals can increase the intrinsic raw material component of the cost by an order of magnitude or more.
Therefore, any improvements made would have to give due consideration to metal costs and the attendant handling issues.
As a manner of addressing the issues of improving performance, lowering cost, ease of manufacturing and flexibility in application, it was important to take a practical approach to selecting the alloying elements for use in the present invention. As was mentioned, the traditional alloying elements of gold, platinum and palladium, are very expensive and are difficult to recover from spent targets and associated scrap. Selecting less expensive and readily available elements that would alloy well with silver, and be readily available in the purities in passivation or inertness to the operating environment was an important step in formulating the alloys of the present invention.
There are several specialty applications in the industry that require reflective or semi-reflective coatings or layers. These include optical storage media, low emissivity glass, transparent conductive displays, and electro-chromic mirrors. The present invention provides useful alloy coating compositions for such applications, and other applications requiring reflective and semi-reflective properties.
Optical discs are commonly used for recording data, video, audio, etc. The discs are usually constructed in four layers (conventional, prerecorded, optical discs). The first layer is typically constructed from optical grade, polycarbonate resin, and manufactured by techniques well-known in the art, usually by injection or compression molding the resin into a disc. The surface of such a disc is molded or stamped with precisely located pits and lands having a predetermined size which store information on the disc.
After stamping (or molding), an optically reflective layer is disposed on the information pits and lands, which is usually between about 40 to about 100 nanometers (nm) thick. Deposition techniques such as sputtering or thermal evaporation are well-known in the art. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed. Vol. 10, pp. 247 to 283, gives a detailed explanation of deposition techniques such as sputtering, thermal evaporation, flow discharge, ion plating, and chemical vapor deposition.
Next, a solvent-based or a UV (ultraviolet) curing-type resin is applied over the reflective layer. This third layer protects the reflective layer from handling and the ambient environment. An optional label identifies the particular information that is stored on the disc, and sometimes, may include artwork.
The information pits, found between the polycarbonate resin and the reflective layer, usually form a continuous spiral. The spiral typically begins at an inside radius and ends at an outside radius. The distance between any 2 spirals is called the “track pitch” and is usually about 1.6 microns. The length of a pit or land is from about 0.9 to about 3.3 microns. (All of these specifications were first proposed by Philips NV of Holland and Sony of Japan as standards for the industry.)
Reading of the disc is accomplished by pointing a laser beam through the optical grade polycarbonate and onto the reflective layer with sufficiently small resolution to focus on the information pits. The pits have a depth of about ¼ of the wavelength of the laser light, which has a wavelength in the range of about 780 to 820 nanometers. Destructive (dark) or constructive (bright) interference of the laser light is then produced as the laser travels along the spiral track, focusing on an alternating stream of pits and lands in its path.
This change of light intensity from dark to bright or from bright to dark forms the basis of a digital data stream of one's and zeros. When there is no light intensity change in a fixed time interval, the digital signal is “0,” and when there is a light intensity change from either dark to bright or bright to dark, the digital signal is “1.” The continuous stream of ones and zeros is then electronically decoded into a meaningful format, such as music.
As a result, it is important to have a highly reflective coating on the disc to reflect the laser light from the disc and onto a detector in order to read the presence of an intensity change. Typically, a reflective layer is copper, silver, aluminum, or gold, all of which have a high optical reflectivity of generally more than 80 percent. Aluminum and aluminum alloys are most commonly used given their easy placement onto a polycarbonate disc, lower cost, and corrosion resistance.
Organic dye is the key to a CD-R disc. The dye is made from solvent and organic compounds from the cyanine, phthalocyanine or azo family. It is normally applied by spin coating onto the disc. A reflective layer is then applied over the dye. Because the dye may contain halogen ions or other chemicals that can corrode the reflective layer, many commonly used reflective layer materials (e.g., aluminum) may not be suitable for use on a CD-R disc. As a result, gold is often used as the reflective layer; however it is a very expensive solution.
Another type of optical disc is a prerecorded digital video disc, “DVD.” This disc comprises two halves, each made of polycarbonate resin and coated with a reflective layer, as described above. The halves are then bonded with a UV curing resin or a hot melt adhesive to form the whole disc. The disc can then be played from both sides. The size of a DVD is about the same as a CD, but the information density is higher, having a track pitch of about 0.7 micron with the length of the pits and lands from approximately 0.3 to 1.4 microns.
One variation of the DVD family of discs is the DVD-dual layer disc which has two information layers. On this disc, the highly reflectivity layer is usually the same as others, but a second layer is only semi-reflective with a reflectivity in the range of approximately 18 to 30 percent. This second layer must also allow a substantial amount of light to pass through, so that the laser beam can reach the highly reflective layer underneath and then reflect back through the semi-reflective layer to the signal detector.
Details regarding the manufacture and construction of DVD discs can be found in U.S. Pat. No. 5,640,382, entitled “Dual Layer Optical Medium Having Partially Reflecting Metal Alloy Layer,” to Florezak et al., issued Jun. 17, 1997.
Additional manufacturing and operating details of an optically readable storage system can be found in U.S. Pat. No. 4,998,239, entitled “Optical Information Recording Medium Containing a Metal Alloy as a Reflective Material,” to Strandjord et al., issued Mar. 5, 1991 and U.S. Pat. No. 4,709,363, entitled “Optically Readable Information Disc Having a Reflection layer Formed From a Metal Alloy,” to Dirks et al., issued Nov. 24, 1987.
Another disc in the compact disc family that has become popular is the recordable compact disc or “CD-R.” This disc is similar to the CD described earlier, with a few minor changes. The recordable compact disc begins with a continuous spiral groove instead of a continuous spiral of pits and has a layer of organic dye between the polycarbonate substrate and the reflective layer. The disc is recorded by periodically focusing a laser beam into the grooves as the laser travels along the spiral track. The laser heats the dye to a high temperature, which in turn places pits in the groove that coincide with an input data stream of ones and zeros by periodically deforming and decomposing the dye. Additional details can be found in U.S. Pat. No. 5,325,351, entitled “Optical Recording Medium Having a Reflective Layer Made of Cu—Ag or Cu—Au Alloy,” to Uchiyama et al., issued Jun. 28, 1994; U.S. Pat. No. 5,391,462 issued Feb. 21, 1995, U.S. Pat. No. 5,414,914 issued May. 16, 1995 and U.S. Pat. No. 5,419,939 issued May 39, 1995, entitled “Optical Recording Disk,” to Arioka et al.; and U.S. Pat. No. 5,620,767, entitled “Light Reflecting and Heat Dissipating Material and Optical Information Recording Medium Using the Same,” to Harigaya et al., issued Apr. 15, 1997.
The typical choice of a semi-reflective layer is gold or silicon in the thickness range of 5 to 70 nanometers, as discussed in U.S. Pat. No. 5,171,392, to Lida et al. Gold, when sufficiently thin, will both reflect and transmit light, has outstanding corrosion resistance, is relatively easy to sputter into a coating of uniform thickness, and is more expensive than other metals. Silicon is a reasonable alternative to gold, but because it is a semiconductor, its sputtering yield and sputtering rate is significantly lower than gold. Silicon also has a tendency to react with oxygen and nitrogen during sputtering. Nevertheless, silicon is useful as an optional component in the alloy of the present invention.
Generally, for aesthetic reasons, a gold or copper based alloy is used to offer the consumer a “gold” colored disc. Although gold naturally offers this rich color and satisfies all the functional requirements of a highly reflective layer, it is more expensive than aluminum. Examples of patents disclosing such gold alloys are: U.S. Pat. No. 5,093,174, entitled “Optical Recording Medium,” to Suzuki et al., issued Mar. 3, 1992, which discloses a metal reflecting layer of an aluminum or silver alloy containing gold for optical recording media; U.S. Pat. No. 6,292,457 B1, entitled “Recordable Optical Media With A Silver-Gold Reflective Layer,” to Preuss et al., issued Sep. 18, 2001, which discloses an optical recording media having a transparent substrate and a reflective layer containing gold; U.S. Pat. No. 6,007,889, issued Dec. 28, 1998; U.S. Pat. No. 6,280,881, issued Aug. 28, 2001; U.S. Pat. No. 6,541,402, issued Sep. 17, 2002; and U.S. Pat. No. 6,544,616 issued Apr. 8, 2003; and U.S. Patent Application Nos. US2002/0034603 filed Apr. 13, 2001 and US2002/0122913 filed Sep. 5, 2002, entitled “Metal Alloys for the Reflective or Semi-Reflective Layer of An Optical Storage Medium,” to Nee, which disclose a silver-based or copper-based alloy thin film for a coating layer for optical discs. The Nee additions to the silver alloy are gold, palladium, copper, rhodium, ruthenium, osmium, iridium, platinum, zinc, aluminum, zinc plus aluminum, manganese, and germanium. The Nee additions to the copper alloy are manganese, silver, cadmium, gold, magnesium, aluminum, beryllium, zirconium and nickel. These patents and applications do not disclose the alloy coatings of the present invention.
Other expensive materials, such as palladium have also been used in the art to produce optical storage media, such as disclosed in: U.S. Pat. No. 6,228,457 B1, entitled “Optical Data Storage Medium,” to Ueno et al., issued May 8, 2001, which discloses an optical data storage medium with a silver-palladium-copper alloy or silver-palladium-titanium alloy; and U.S. Pat. No. 6,242,068, entitled “Recordable Optical Media with a Silver-Palladium Reflective Layer,” to Preuss, issued Jun. 3, 2001, which discloses a reflective layer made of silver and palladium. The patents do not disclose the alloy coatings of the present invention.
A copper-based alloy that contains aluminum, zinc or tin is sometimes used to produce a “gold” looking layer. However, alloys of copper corrode more easily than aluminum.
U.S. Pat. No. 6,351,446, issued Feb. 26, 2003, and U.S. Patent Application No. US2002/0054973, filed Nov. 26, 2001, entitled “Optical Data Storage Disk,” to Weinzerl, disclose an optical data storage disk with at least two interfaces. The inner layer is the reflection layer and the other layer is a partially reflecting/partially transmitting layer. The inner layer is made of one type of alloy and the other layer is made of another alloy. The Weinzerl patent and application do not disclose the alloy coatings of the present invention.
Several silver-based alloys have been developed to improve tarnish resistance in multi-layer stacks. Although silver-based alloys are commonly used in the casting industry (e.g. for jewelry making), they have not heretofore been utilized as reflective or semi-reflective coatings for specialty applications, including, but not limited to, optical storage media, low emissivity glass, transparent conductive displays, electro-chromic mirrors, and other reflective or semi-reflective applications. As indicated above, these silver-based alloys have typically included gold or palladium, very expensive components. These alloys traditionally have had 80% to 95% silver and employed gold or platinum group metals as alloying elements to stabilize the properties of the silver when exposed to moisture or mildly acidic environments.
The present invention is a new, lower cost alloy coating, specifically useful for optical storage media, low emissivity glass, transparent conductive displays, electro-chromic mirrors, reflective applications, semi-reflective applications and highly reflective applications that represent a favorable balance between cost and performance. The preferred alloy of the present invention is more complex than the standard binary or ternary alloys presently known in the art, however, it can be produced using readily available production equipment.